1. Field of the Invention
The invention relates to a lens, and more particularly to a projection lens.
2. Description of the Related Art
Various lenses are disclosed in U.S. Pat. No. 4,993,814, U.S. Pat. No. 5,233,474, U.S. Pat. No. 5,796,528, U.S. Pat. No. 5,251,073, U.S. Pat. No. 6,747,816, US publication No. 2008/0266678 and US publication No. 2003/0161050. However, a projection lens applied to a projection apparatus may have high imaging quality. In general, a frame projected by the projection lens may satisfy multiple conditions such as low distortion aberration, high resolution, high contrast, and high uniformity, etc. Furthermore, for projecting a larger frame in a shorter projection distance, the projection lens may have a wide field of view (FOV) and low keystone aberration. Moreover, for increasing light utility efficiency and brightness uniformity of the projected frame, a telecentric angle, that is a maximum angle of a chief ray at a reducing side related to an optical axis of the projection lens, may be small enough to enable the chief ray to be substantially parallel with the optical axis.
It needs to overcome multiple difficulties to design a projection lens which satisfies the above-mentioned conditions. For example, for reducing the distortion aberration, the FOV may be reduced and numbers of the lenses of the projection lens may be increased. For achieving features of large FOV and small telecentric angle, a total length of the projection lens and sizes of the lenses would be increased. In conventional technique, a projection lens usually includes at least six lenses to achieve features of effective focal length being about 10 mm and imaging quality satisfying the above-mentioned conditions. Therefore, the conventional projection lens can not achieve a compact size and low cost.
Other objectives, features and advantages of the present invention will be further understood from the further technological features disclosed by the embodiments of the present invention wherein there are shown and described preferred embodiments of this invention, simply by way of illustration of modes best suited to carry out the invention.